X-Ray Computed Tomography Scanning for Defense Applications

Source: NTS

X-ray computed tomography (XCT) scanning has been used for decades and remains a key component in nondestructive testing (NDT). XCT allows a user to create a three-dimensional (3-D) representation of a scanned object within a computer software program and highly detailed output files for future use and manipulation. XCT has evolved over recent years for research and development, reverse engineering, manufacturing and quality control, and life-cycle management. For defense applications, XCT has been very useful in supporting ballistic test and evaluation, armor design and testing, parts inspection, reverse engineering, and evaluating additively manufactured parts.

In many industries, XCT applications can include the following:

Composite analysis

Crack inspections

Dimensional inspections

Failure analysis

Manufacturing flaw detection

Foreign object damage and inclusion

Additive manufacturing inspection

Nondestructive internal measurement

Microporosity studies

Nonuniformity studies

Porosity Inspections

Part-to-computer-aided design (CAD) inspection and digitization

Wall-thickness analysis

Counterfeit electronics analysis

Images captured by XCT can show many external and internal component features that can be used for further analysis or remodeling efforts. Depending on the software and level of resolution supporting each XCT scanner, the resultant scans will show detailed geometries, internal structures, component density variations, defects, and more.

How It Works

Computed tomography operates by using an X-ray source and X-ray detectors that capture a series of two-dimensional (2-D) images. The emitter focuses a narrow beam of X-rays toward and through an object. As the X-rays exit the component, they are picked up and digitized by the detectors on the far side. Each X-ray emission/detection captures a 2-D projection, called a tomography image. A value for the calculated density of material is assigned to each 2-D pixel. Since a 3-D image is sought, the emitter/detector must either rotate around the object components (as a medical computed tomography [CT] scanner at a healthcare facility operates), leaving the object at rest, or the component rotates in between the emitter/detector (as a typical industrial CT scanner operates). The collection of the 2-D projections forms a digitized 3-D object (i.e., tomographic reconstruction) that can be viewed, analyzed, and/or outputted for further analysis and remodeling. At this point, each voxel (3-D pixel) in the dataset carries the calculated density of the component in 3-D space.

The overall resolution of XCT scanning is heavily influenced by the x-ray focal spot size. A smaller spot size means more defined or sharper edges and smaller detected features and allows XCT to become more of a metrology tool. Other conditions which govern the resolution are the amount of power elicited for each scan, as seen in Figure 1. Ryan Peitsch of NTS Chesapeake stated, “Higher resolution is easily obtainable on smaller samples. The larger the sample, the further we must zoom out and the more energy we need; therefore, the larger the focal spot, the longer the focal distance. We can change the focal point based on the size of the sample. The energy determines the focal spot size and the resolution you need will determine the focal length (i.e., zoom). Using 100-kV and 50-W as our settings results in a different focal size than 100-kV and 200-W as our settings. The rule of thumb is that the focal spot size scales with your energy and Watts” [1]. In addition, the closer you can bring the sample to the X-ray source, the larger the results will be projected on the detector panel, as seen in Figure 2. This results in higher resolution scans, but only for smaller samples. For example, the large 36‑ × 36‑inch X-ray detector panel used by NTS’s custom Nikon 225/450kV XCT system can achieve resolutions to nearly 1 µm for very small polymer and even aluminum parts.

Figure 1: Illustrations of XCT Focal Points (Source: NTS).

Figure 2: Resolution vs. Sample (Source: NTS).

Applications

Research and Development (R&D)

The focus of R&D efforts can include fiber orientation/analysis, material porosity, geometric analysis, or the verification and validation of specific models. Examples of typical R&D efforts for composite materials can include carbon prepreg analysis, fiber volume fraction, resin volume, or a composite analysis scan. A high-resolution composite analysis scan can show its user if single fiber strands are found on a surface not congruent with a carbon block, as shown in Figure 3.

Figure 3: Detail of Carbon Block (Source: NTS).

Further examples of R&D include material porosity. XCT scanning can show its user the results of die‑cast defects resulting in excessive porosities, inconsistent material properties in injection molded or additively-manufactured polymers, errors in internal geometries for additively-manufactured metals, or transitions between thick and thin walls from machining or welding (see Figure 4). XCT can be an important NDT method used to identify problem areas before a failure occurs or before a full production process begins. Specific to defense and law enforcement applications, XCT is used widely for investigating the effectiveness of armor (vehicle, body, helmet, or otherwise) designs. XCT allows government agencies and materiel developers to nondestructively evaluate internal damage caused by impacting threats on composite, ceramic, transparent, and even metallic armor designs. This can be especially useful for determining the best approach for multihit protection requirements and optimizing armors for weight savings.

Figure 4: Crack, Porosity, and Void Detection (Source: NTS).

Lifecycle Management (LM) and Reverse Engineering

LM is the process of managing the whole life cycle of a product from inception through its design, manufacturing, service, and disposal. Often within the U.S. Department of Defense, the service lives of specific platforms are extended far beyond their planned life cycles rather than starting up new programs. When this extension occurs, part and supplier obsolescence generally become an issue. Replacement parts for a platform become difficult to find and procure due to a lack of stocking or abandoned manufacturing processes and businesses. XCT scanning is one possible avenue for digitizing existing parts and recreating lost 3-D geometry that can be used for redesign, reverse engineering, and manufacturing. It is especially well-suited for developing surface geometries from complex shape parts such as faceplates, bezels, etc. Using XCT scans instead of laser, structured light, or touch probe measurements, all geometries can be captured in a single acquisition without line-of-sight issues inherent to traditional scanning/digitization methods. Combining multiple scans is not needed, reducing post-processing time and errors. Output formats generally include point clouds (*.xyz, *.txt, etc.) or polygons (*.stl). Depending on the complexities of XCT scanning, these outputs can be directly utilized and/or modeled within the end-user’s preferred commercial CAD software or indirectly using additional software where curves, parametric sketches, and other features can be extracted and applied to the aforementioned CAD packages (see Figure 5).

Figure 5: Using the Output From XCT (Left) and Parametric Models Created in Commercial CAD (Right) (Source: SURVICE Engineering).

Manufacturing and Quality Control

Industrial uses for XCT include first-article inspection, failure analysis, wall-thickness analysis, direct-CAD comparisons, and geometric dimensioning and tolerancing. XCT scanning offers very precise measurements. Generally, data sets can be used to generate an analysis of any aspect of a component, both internally and externally. Figure 6 illustrates color-coded parts showing where deviations occur between the scans and manufacturing specifications.

In addition, XCT plays a role in failure analysis and investigative/forensic engineering. XCT scanning can show users in these technical areas specifically where and how components fail. Parts can be nondestructively analyzed to determine how they were manufactured, what materials were used, and how they were assembled. An example of this type of analysis performed on a bullet is shown in Figure 7. Investigations involving battery failures have become very common in the past decade. In 2013, NTS Chesapeake’s Non-Destructive Testing Division was contracted by the National Transportation Safety Board (NTSB) to determine if internal damage had occurred within lithium-ion battery units for several high-profile incidents related to aircraft safety [1]. Their evaluation for the extent of damage included thermal damage, deformation, proof of punctures, etc. (Figure 8).

Conclusions

CT scanning began in the medical field with the capacity to scan a human head and relatively low‑density, organic materials. Today, the technology has expanded to include the ability to scan envelopes of 36 × 36 inches (27 ft3) at resolutions of 150–400 µm and even volumes the size of military vehicle hulls at somewhat lower resolutions. The high power of industrial XCT systems also allows scanning higher density materials such as ceramics, glasses, and metals. As newer technology is developed, XCT scanning is constantly improving its capability in resolution, scanning size, materials, and software support.

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